The present invention generally relates to an ignition system for an internal combustion engine and more particularly, to an improved construction for connecting an ignition coil and a spark plug in the ignition system.
In a known ignition system for an internal combustion engine, a tubular terminal 2A or 2B is mounted in an upper portion of a through-hole 1a of a tubular insulating member 1 and an electrode 4 of an ignition coil 3 is fitted into an upper portion of the terminal 2A or 2B so as to be electrically connected to the terminal 2A or 2B as shown in FIG. 1 or FIG. 2. Meanwhile, a coiled spring 5 attached to a lower portion of the terminal 2A and 2B is loosely fitted into a lower portion of the through-hole 1a of the insulating member 1. Furthermore, an electrode 7 of a spark plug 6 is fitted into the through-hole 1a from a lower end of the insulating member 1 and is brought into contact with the coiled spring 5 so as to be electrically connected to the terminal 2A or 2B. Meanwhile, in FIGS. 1 and 2, reference numeral 8 denotes a mounting hole formed in a cylinder head of the engine. In the known ignition system, a distance L1 or L2 (L1&lt;L2) between the electrode 4 of the ignition coil 3 and the electrode 7 of the spark plug 6 varies according to types of the internal combustion engine. In case the distances L1 and L2 are different from each other as described above, lengths H1 and H2 (H1&lt;H2) of the insulating member 1 are also different from each other and a length H3 of the terminal 2A is also different from a length H4 (H3&lt;H4) of the terminal 2B. Therefore, in accordance with variations of the distance L1 or L2 between the electrode 4 of the ignition coil 3 and the electrode 7 of the spark plug 6, different molding dies for producing the insulating member 1 and the terminal 2A or 2B having different lengths should be prepared, thereby resulting in increased production cost of the ignition system. Meanwhile, since kinds of the components of the ignition system increases, control of the components becomes complicated, so that such a possibility exists that wrong components are used for the ignition system, thereby resulting in deterioration of reliability of the ignition system.
In order to obviate this problem, it may be considered that the terminal 2A having the length H3 shorter than the length H4 of the terminal 2B is used in common for the lengths L1 and L2 between the electrode 4 of the ignition coil 3 and the electrode 7 of the spark plug 6 and difference between the lengths L1 and L2 is eliminated by changing compression amount of the coiled spring 5 such that drop of production cost of the components and facilitation of control of the components are pursued. However, if the amount of compression of the coiled spring 5 changes, force at which the coiled spring 5 is brought into contact with the electrode 7 of the spark plug 6, namely, reaction force of the coiled spring 5 changes and thus, it is preferable that spring constant of the coiled spring 5 is as small as possible. In order to make the spring constant of the coiled spring 5 small, wire diameter of the coiled spring 5 may be reduced or coil diameter of the coiled spring 5 may be increased. However, in case the wire diameter of the coiled spring 5 is reduced, efficiency for mounting the coiled spring 5 deteriorates. On the other hand, if the coil diameter of the coiled spring 5 is increased, it may be impossible to fit the coiled spring 5 into the terminal 2A.
As a result, the coiled spring 5 should have small spring constant and small coil diameter. In this case, ratio of longitudinal length to lateral length of the coiled spring 5, i.e., ratio of free height to mean coil diameter of the coiled spring 5 increases. This ratio preferably ranges from 0.8 to 4 generally. If the ratio exceeds this range, such a problem arises that when the spark plug 6 has been fitted into the through-hole 1a of the insulating member 1, buckling of the coiled spring 5 is likely to take place as shown by the two-dot chain lines in FIG. 3.